Internal focusing telephoto lens

Abstract

The internal focusing telephoto lens includes a first lens group I of positive refractive power, a second lens group II of negative refractive power and a third lens group III of positive refractive power, arranged in order from an object side. The second lens group II is movable along an optical axis for focusing. The first lens group I includes a first positive lens I-1, a second positive meniscus lens I-2 whose object side surface is convex, a third positive meniscus lens I-3 whose object side surface is convex and a fourth negative meniscus lens I-4 whose object side surface is convex, in order from the object side. At least two of the first positive lens I-1, the second positive meniscus lens I-2 and the third positive meniscus lens I-3 in the first lens group I satisfy the following conditions (1) and (2);where nI is refractive index and νI is Abbe number of the lenses in the first lens group I.

What is claimed is:  
     
       1. An internal focusing telephoto lens having a plurality of lens groups arranged in order from an object side, said telephoto lens comprising: 
       a first lens group I of positive refractive power, said first lens group I comprising a first positive lens I-1, a second positive meniscus lens I-2 whose object side surface is convex, a third positive meniscus lens I-3 whose object side surface is convex and a fourth negative meniscus lens I-4 whose object side surface is convex, in order from said object side;  
       a second lens group II of negative refractive power, said second lens group II being movable along an optical axis for focusing; and  
       a third lens group III of positive refractive power, said third lens group III comprising a first meniscus lens III-1 whose object side surface is concave and a second positive lens III-2;  
       wherein at least two positive lenses in said first lens group I satisfy the following conditions (1) and (2);  
       
         
           1.55 <n   I <1.75  (1)  
         
       
       
         
           61<ν I   (2)  
         
       
       where n I  is refractive index and ν I  is Abbe number of the lenses in the first lens group I.  
     
     
       2. The internal focusing telephoto lens according to claim  1 , wherein said first lens III-1 in said third lens group III consists of a biconcave lens and a biconvex lens that are cemented to each other. 
     
     
       3. The internal focusing telephoto lens according to claim  1 , further satisfying the following condition (3); 
       
         
           −10 <f/r   III-1-1 <−1.5  (3)  
         
       
       where f is focal length of the entire lens system in an infinite focusing condition and r III-1-1  is a radius of curvature of the object side surface of said first lens III-1 in said third lens group III.  
     
     
       4. The internal focusing telephoto lens according to claim  1 , further satisfying the following condition (4); 
       
         
           0.003 <d   III-1-III-2   /f< 0.25  (4)  
         
       
       where f is focal length of the entire lens system in an infinite focusing condition and d III-1-III-2  is a distance between said first lens III-1 and said second lens III-2 in said third lens group III.  
     
     
       5. An internal focusing telephoto lens having a plurality of lens groups arranged in order from an object side, said telephoto lens comprising: 
       a first lens group I of positive refractive power;  
       a second lens group II of negative refractive power, said second lens group II being movable along an optical axis for focusing, wherein said second lens group II consists of a single lens element; and  
       a third lens group III of positive refractive power.  
     
     
       6. The internal focusing telephoto lens according to claim  5 , wherein said first lens group I comprises a first positive lens I-1, a second positive meniscus lens I-2 whose object side surface is convex, a third positive meniscus lens I-3 whose object side surface is convex and a fourth negative meniscus lens I-4 whose object side surface is convex, in order from said object side, and wherein said third lens group III comprises a first meniscus lens III-1 whose object side surface is concave and a second positive lens III-2. 
     
     
       7. The internal focusing telephoto lens according to claim  6 , wherein at least two positive lenses in said first lens group I satisfy the following conditions (1) and (2); 
       
         
           1.55 <n   I <1.75  (1)  
         
       
       
         
           61<ν I   (2)  
         
       
       where n I  is refractive index and ν I  is Abbe number of the lenses in the first lens group I.  
     
     
       8. The internal focusing telephoto lens according to claim  6 , further satisfying the following condition (3); 
       
         
           −10 <f/r   III-1-1 <−1.5  (3)  
         
       
       where f is focal length of the entire lens system in an infinite focusing condition and r III-1-1  is a radius of curvature of the object side surface of said first lens III-1 in said third lens group III.  
     
     
       9. The internal focusing telephoto lens according to claim  6 , further satisfying the following condition (4); 
       
         
           0.003 <d   III-1-III-2   /f< 0.25  (4)  
         
       
       where f is focal length of the entire lens system in an infinite focusing condition and d III-1-III-2  is a distance between said first lens III-1 and said second lens III-2 in said third lens group III.  
     
     
       10. The internal focusing telephoto lens according to claim  5 , further satisfying the following condition (5); 
       
         
           65<ν II   (5)  
         
       
       where ν II  is Abbe number of said single lens element of said second lens group II.